Synchronous motor protective system operative on pull-out



March 24, 1953 T. F. BELLINGER 2,632,875

SYNCHRONOUS MOTOR PROTECTIVE SYSTEM OPERATIVE ON PULL-OUT Filed June 8,1950 Patented Mar. 24, 1953 SYNCHRONOUS MOTOR PROTECTIVE SYS- TEMOPERATIVE N PULL-OUT Thaddeus Bellinger, West Allis, Wis, assignor toAllis-Chalmers Manufacturing; Company,

lviiiwaukee, Wis.

Application June. 8', 1950, Serial No. 166,935

12. Claims. (01. 318167) This invention relates-in general tosynchronous motor control systems and inparticular to systems forcontrolling the removal ofv the fie d excitation of the motor when themotor pulls out of synchronism.

In prior art systems of this type, a field application relay having acurrent winding traversed by the current induced in the field winding ofthe motor is utilized-to connect the field winding to a source of directcurrent excitation when the motor approachessynchronous speed Most ofsuch systems also rely on the current winding of the field applicationrelay to detect currents induced in the field winding. when the motorpulls out of synchronism, for disconnecting the field winding from itssource.

Such systems operate satisfactorily for motors in which the differencein magnitude between the induced current flowing during synchronizationand the normal unidirectional excitation currentis not great. However,in some motors, the difference in magnitude mentioned above is so largeas to render exceedingly difficult the design of one field applicationrelay which is sufficiently sensitive for both values of current, owingto the saturation of the relay core by the large unidirectional currentflowing in the field during synchronous operation and the dificulty ofbuilding arelay dimensioned for such current.

This disadvantage can be overcome by using a field application relay fordetermining the point at which held excitation is tobe applied and byusing a separate electroresponsive device for detesting pull-cut. Suchelectroresponsive device may be an alternating current transformer having a primary winding traversed by the current flowing in the motorfield winding and having a sensitive relay inductively coupled. theretofor energizaticn in response to induced currents oi" predeterminedmagnitude flowing in the primary Winding. Also, a reactor havingacurrent wind! ing connected in the circuit of the motor field windingand having a relay conductively coupled thereto. may be utilized. Tooppose the saturat-- ing effect of the unidirectional field current, thetransformer or reactor may be equipped with a winding supplied withcurrent in a direction opposing the flux produced by the field current,or it may be provided with air gaps in its core. Operation of thesensitive relay may be utilized to disconnect the field Winding from thedirect current source for resynchronizing or to operate an alarm 01' to.disconnect the motor armature from its source.

It is therefore an object of this invention. to provide an improvedsynchronous motor protective system which will automaticallyremovexfield excitation if the motor pulls out of step.

It is afurther object of thisinvention to pro.

vide a system for controlling disconnection of the field winding of asynchronous motor from its source under certain conditions, in which thedisconnecting means are rendered unresponsive:

for a predetermined time.

It is a further object of this invention to provide a synchronous motorprotective system uti-. lining an electroresponsive device in which anelement of the device is utilized to prevent saturation of the device.

Objects and. advantages other than those cute lined above will bereadily apparent from the,

following detailed. description. when read. in con.- nection with theaccompanyingdrawingin which:

Fig. l diagrammatically illustrates theconneoe tions and apparatusembodied in one. form of the. invention; and

Figs. 2, 3 and lpartly illustrate modifications of the connections. andapparatus. illustrated in Fig. 1.

In the embodiment illustrated in Fig. l a syn! chronous. motor 2 is.shown as provided with a.

field windingB. which may be energized from any suitable source ofdirect. current such as an ex-.- citer 4. Exciter lmay be driven by anysuitable means such as. the separate motor 4. Field Winding S'may beconnected to and disconnectedfrom exciter 4 by suitable switching meanssuch as contacts 50 and 5c of a field contactor. 5.

The connection of field winding 3. to. exciter 4 by cont-actor 5 iscontrolled through a suitable.

field application relay 6. Relay 5 is preferably of the type disclosedin U. S. 2,478,693, granted to William J. Herziger, and comprises adirect current winding 5 energized from asuitable. direct current sourcesuch as a battery-9, andnanraltere hating currentwinding ii. Withcontactor 5. the uDsition shown, winding 9 is connected; in series withfield winding 3 and a suitablefield,

dischar e. resistor it through contacts 5d of'contactcr 5, to be therebyenergized by a current proportional to the current induced in fieldwind-u. ing 3 by the motor armature. Windings; land 8.

are wound upon a generally Eeshaped' core at the open end of'which anarmature I l is mounted on a pivot l2 and biased by gravity to the posistion shown to close a pair of" contacts. 13'. Armae.

ture 5! is attracted by the resultant flux of:

windings l and 8, andthe value of the resultant flux required to retainarmature I I attracted may be readily varied by adjustment of theposition of a nonmagnetic block Hi adjustablymounted on the E-shapedcore and. extending 3 more or less beyond the end face of the upper coremember.

The disconnection of field winding 3 from exciter A is controlled bymeans including an electroresponsive device such as an alternatingcurrent transformer ll having a primary winding 18, a secondary winding:9, a magnetic core as and a biasing Winding 2i.

Primary winding is is connected in series with field winding 3 whencontacts 50 and 5e are closed to thereby supply to Winding i8 a currentproportional to the current induced by the armature winding in fieldwinding 3. Secondary winding I9 is connected to the coil 22a of apull-out current relay 22, thereby inductively coupling relay 22 toprimary winding i8. Biasing winding 2! is connected to a suitable sourceof direct current, such as exciter 5, through an adjustable resistor 23and is thereby supplied with an adjustable constant direct current whenexciter is running.

Initiation of operation of the system is under the control of a masterrelay 2s having an energizing coil 25a. The energization circuit of coil25a may be traced from one side of battery 9, through a startingpushbutton switch 2'5, coil 26a, a stop pushbutton switch 28, contacts22b of relay 22, back to the other side of battery 53. Contacts 262) ofrelay 26 bridge switch 2? to provide sealing in of relay 26 uponenergization. Contacts 260 when closed complete the energizing circuitthrough battery 9 for a coil 29a of a main line contactor it havingcontacts 2% for connecting the armature winding of motor 2 to a suitablesource of alternating current represented by conductors 35. Contacts 256connect winding I of field application relay 6 across battery 9 andcontrol the energization circuit of coil 5a of contactor 5. Contacts 26dcomplete the energization circuit of the coil 32a of a timing relay 32having contacts 3%.

Relay 32 is of the type involving a time delay only upon energization,which time delay is obtained by any suitable means such as a dashpot32c. Contacts 321) are connected in the circuit of the coil 33a of anauxiliary relay 33 having contacts 33b, 330. The energizing circuit ofcoil 3311 may be traced from the right side of battery 9 through coil33a, contacts 32b, a conductor 34, through contacts 53 of fieldapplication relay 6, and through a conductor 36 back to battery 9.Contacts 331) are connected by two conductors 31, 38 to the conductors3%, as to thereby seal in relay 33 across contacts l3. Closure ofcontacts 33c connects the coil Ma of a polarized relay 4l throughconductors d2, 53 across an adjustable portion of field dischargeresistor i8. Relay ll is polarized by virtue of the connection of arectifier 49 in series with coil Ma. Relay M is provided with contactsMb which connect the energizing coil 46a of a second timing relay 46across battery 9,

- Relay 46 is provided with normally open contacts 46b and normallyclosed contacts 460, and is of the type involvin a time delay only upondeenergization, which time delay may be obtained by a dashpot ifib.Contacts Mb connect the coil 41a of a relay l! across battery 9.Contacts 41b seal in coil 4M across contacts 46b, while contacts tlc andite complete the energizing circuit of coil 5a of field contactor 5through a circuit which may be traced from the right side of battery 9,through contacts 410, ite, a conductor 48, coil 5a, conductor 49 andthrough contacts 26c back to battery 9.

.Contacts 512 of contactor 5 control the energization of the coil 5la ofa relay 5| provided with contacts Mb. The circuit for coil 5m may betraced from the left side of battery 9 through contacts 26c, conductor49, a conductor 52, coil 51a, contacts 517, conductor 38, contacts 410,450, back to battery 9. Relay 55 is also provided with a dashpot die toprovide a time delay in the operation of relay 5! only upon energizationof coil em.

Starting of motor 2 is initiated by closing pushbutton switch 2'],thereby energizing coil 26a of master relay 25. Contacts 2619 close toseal in relay 26 and contacts 250 close to complete the energizingcircuit of coil 29a of contactor 29, thereby connecting the armaturewinding of motor 2 to conductors 31. The armature of motor 2 isenergized and induces in field winding 3 a'current having a magnitudeand frequency inversely proportional to the speed of retation. Thiinduced current also flows through discharge resistor lil, contacts 5dand winding 8 of field application relay 6.

Contacts Elie close to energize winding 7 of field application relay 6from battery 9. Contacts 26d close to energize coil 32a of relay 32, andrelay 32 closes its contacts 321) in the circuit of coil 33a after apredetermined time delay. This time delay is of sufiicient length topermit the current in winding 8 to produce suificient flux to attractarmature l I, thereby opening contacts l3. Opening of contacts i3 breaksthe energizing circuit of coil 33a, and relay 33 consequently remainsdeenergized.

As the speed of motor 2 increases and approaches synchronous speed, thefrequency and magnitude of the current induced in field winding 3decrease. At an adjustable predetermined value of current in winding 8,the resultant flux in relay 6 remains for a sufiicient length of timebelow the value necessary to retain armature ll attracted, and thearmature drops out to close contacts 3. The direction of current throughwinding 1 is so chosen that the resultant fiux in relay 6 becomesinsufficient to retain armature ii during the positive half cycle of thecurrent induced in field winding 3. Closure of contacts l3 completes theenergizing circuit for coil 33a of relay 33-, which closes its contacts330 to seal itself in across contacts i3. Contacts 33d also close toconnect coil lla of polarized relay 4| across a portion of the fielddischarge resistor It. Relay ii is so poled as to conduct current onlyduring the positive half cycles of the current flowing in resistor It,and therefore remains deenergized during the negative half cycle ofcurrent in resistor iii immediately following closure of contact 53.

On commencement of the positive half cycle following the positive halfcycle during whichcontact i3 closed, relay ii is supplied withsufiicient current to close its contacts ii I) and thereby energize coilMia. Relay 46 immediately closes its contacts 462?, and opens itscontacts 46c, contacts ifib completing the energizing circuit of coil.

47a and contacts 560 breaking the energizing circuit for coil 5a.Energization of coil 41a causes contacts Mb to close to seal in relay 4!and causes contacts 410 to close in the circuit of coil 5a. However,coil 50. remains deenergized owing to the opening of contacts 480described above.

On the negative half cycle of current in resistor ill followingenergization of polarized relay ll, relay 4! becomes deenergized andopens its contacts Mb to deenergize coil 46a. After'a predeterminedtime, relay 46 drops out to close contacts 450- to complete theenergizing circuit for coil 5a. Contactor 5 then opens its contacts 5dto disconnect field winding 3 from discharge resistor iii, and closesits contacts 50 and 52 to connect field winding 3 to exciter d, inseries with primary winding i8 of transformer ii. The exact point in thenegative half cycle of current at which field winding 3 is connected toexciter 4 may be readily varied by adjustment of the drop out time ofrelay 35 to secure the most advantageous operation of the system.

To prevent actuation of relay 22 in response to transient currentsinduced in field winding 3 when the motor pulls into synchronism uponconnection of the field winding .3 to exciter 4, contact means areprovided for short circuiting relay 22"ior a predetermined time afterconnection of field 3 to exciter 4. These means include relay coil 5mwhich is energized by contact to of field contactor a, through a circuitpreviously described, upon connection of exciter to field winding 3.Relay 5i has a time delay upon energization, during which time contacts5520 short circuit relay coil 22a to thereby prevent transient currentsinduced in winding 13 from actuating relay 22. After the predeterminedtime, contacts 5i?) open to render relay 22 responsive to the currentfiowing in winding l9. Biasing winding ii of transformer ii is suppliedwith current in a direction to oppose the flux produced in the core oftransform r ii by the unidirectional field current traversing primarywinding 18. Winding 2i thus prevents saturation of the core oftransformer ll, thereby rendering transformer il' very sensitive tocurrents produced by a pullout of motor 2.

If after connection of field winding 3 to exciter i, the motor pulls outor" synchronism, an induced current fiows in field winding 3 and primarywinding 58 of transformer E's, thereby inducing in secondary winding 9 acurrent which is a measure of the current induced in field winding 3.Windings 2i and it are so designed that the time constant of winding i8is considerably less than that of winding 2!, to insure rapid responseof relay 22 upon pull-out of motor 2. If the current induced in winding59 is of suificient magnitude, depending upon the setting of relay 22,relay 22 is actuated to open its contacts 22b in the circuit of coil2511, thereby deenergizing relay 26. Deenergization of relay 2Gdeenergizes coil 28a through contact 260 to disconnect motor 2 fromconductors 3i and deenergizes coil 50: through opening of contact 26a todisconnect field winding 3 from exciter t.

In the modification partly shown in Fig. 2, core 26 of electroresponsivedevice i? is provided with air gaps by insertion of nonmagnetic blocksor shims 55 in the magnetic circuit. The use of the air gap eliminatesthe necessity of using biasing winding 22, since the air gap effectivelyprevents saturation oi the core or" device ill by the unidirectionaifield current flowing in primary winding i3.

Fig. 3 partly shows a modification in which the electroresponsive deviceis in the form of a reactor 58 having a magnetic core 69. Reactor 5% isprovided with a current winding 57 which is connected in the circuit offield winding 3 similarly to winding iii of transformer ll. Coil 22a isconnected directly across current winding 5?. Winding 5'! offers verylittle impedance to the fiow of direct current during normal synchronousoperation of motor 2 and therefore very little current fiows throughcoil 2211 during.

this time. However, when. motor 2- pullso'ut oi:

synchronism, winding 51 presents a substantialimpedance to thealternating current induced in field winding 3, thereby energizing coil22a with a substantial current to cause contacts 22b to close anddisconnect the armature and field winding 3, as hereinbefore described.Reactor 56 may be provided with a biasing winding 58- energized from anysuitable direct currentsource to prevent saturation of the core, or itmay be provided with air gaps as in the modification shown in Fig. 2.

If desired a rectifier 6! may be connected in serieswith coil 22a acrosscurrent winding 5"! of reactor 56.

rent through coil 22a, thereby insuringthat relay 22 is not energized bythe unidirectional current in winding 3 during synchronous operation.

To prevent energization of relay 2i! in response to transient currentsinduced in winding 3 uponfield application, a relay 62 having a coil62a, contacts 52b and a dashpot 620 is provided. Coil 62a is energizedthrough the samev circuit as coil tic in Fig. l, and normally opencontacts 62?) are connected in series with coil 22a and rectifier Bi.Dashpot 620 provides a time delay only upon energization of coil 62a.Thereforawhen coil 62a is energized by operation of field contactor 5,througha circuit'previously described, con acts 622) remain open for apredetermined time, thereby rendering relay 22 unresponsive for apredetermined time following closure of the switching means 5.

Fig. i partly shows a modification of the cir-" cuits in which acapacitor 64 is connected in series with coil 22a across thecurrent'winding 51 of reactor 56. Core Bil of the reactor is providedwith air gaps by inserting nonmagnetic blocks 55 in the magnetic circuitto prevent saturation of the core 56. Capacitor 5d prevents the flow ofnormal unidirectional excitation current through coil 22c, but when themotor pulls out of synchronism, the alternating current induced in fieldwinding 3 will fiow through capacitor 64 and coil 22a to disconnectfield winding 3 from exciter s. Time delay relay 62 is provided toprevent actuation of relay 22 in response to transient currents uponfield application.

Although only a few embodiments of the present invention have been shownand described, it-

will be readily apparent to those skilled in the art that variousmodifications may be made therein without departing from the spirit ofthe invention as expressed in the appended claims.

It is claimed and desired to secure by Letters Patent:

1. In a system comprising a synchronous'motor having an armature windingand a field winding, a source of alternating current, a source of directcurrent, and means for connecting said armature windinggto said sourceof alternating current, the combination of switching means forconnecting said field winding to said direct current source to supplydirect current to said field winding, a first relay responsive to thecurrent induced by said armature winding in said field winding prior toclosure of said switching means for causing closure of said switchingmeans to connect said field winding in circuit with said direct currentsource, an electroresponsive device having a magnetic core and a currentwinding connected in said field winding circuit, a second relay coupledto said current winding and re sponsive to said induced: current afterclosure of The rectifier 6| is poled to prevent the flow of normalunidirectional excitation cur- 7 said switching means for causingopening of said switching means to disconnect said field winding fromsaid direct current source, and means including an element of saiddevice for preventing saturation of said core by said direct current.

2. In a system comprising a synchronous motor having an armature windingand a field winding, a source of alternating current, a source of directcurrent, and means for connecting said armature winding to said sourceof alternating current, the combination of switching means forconnecting said field winding to said source of direct current, a firstrelay responsive tothe current induced by said armature Winding in saidfield winding prior to closure of said switching means for causingclosure of said switching means to connect said field winding in circuitwith said direct current source, an electroresponsive device having acurrent winding connected in said field winding circuit, a second relaycoupled to said current winding and responsive to said induced currentfor causing opening of said switching means to disconnect said fieldwinding from said direct current source, and a third relay energized inresponse to closure of said switching means for rendering said secondrelay unresponsive to said induced currents for a predetermined timefollowing closure of said switching means.

3. In a system comprising a synchronous motor having an armature windingand a field winding, a cource of alternating current, a source of directcurrent, and means for connecting said armature winding to said sourceof alternating current, the combination of switching means forconnecting said field winding to said direct current source to supplydirect current to said field winding, a relay responsive to the currentinduced by said armature winding in said field winding prior to closureof said switching means for causing closure of said switching means toconnect said field winding to said direct current source, means forcausing opening or" said switching means to disconnect said fieldwinding from said direct current source including a current transformerresponsive to current induced in said field winding after closure ofsaid switching means, said current transformer having a magnetic coreand a current winding connected in circuit with said field winding, andmeans including an element of said transformer for preventing saturationor" said core by said direct current.

4. In a system comprising a synchronous motor having an armature windingand a field winding, a source of alternating current, a source of directcurrent, and means for connecting said armature winding to said sourceof alternating current, the combination of switching means forconnecting said field winding to said direct current source, a firstrelay responsive to the current induced by said armature winding in saidfield winding prior to closure of said switching means for causingclosure of said switching means to connect said field winding to saidsource of direct current, a current transformer having a primarywinding, a secondary winding and a biasing winding, means for supplyingto said primary winding a current which is a measure of the currentinduced in said field winding after closure of said switching means,means for supplying to said biasing winding a constant direct current,and a second relay responsive to the current fiowing in said secondarywinding for causing opening of said switching means to disconnect saidfield winding from said direct current source,

5. In a system comprising a synchronous mctor having an armature windingand a field winding, a source of alternating current, a source of directcurrent, and means for connecting said armature winding to said sourceof alternating current, the combination of switching means forconnecting said field winding to said direct current source, a firstrelay responsive to the current induced by said armature winding in saidfield winding prior to closure of said switching means for causingclosure of said switching means to connect said field winding to saiddirect current source, a current transformer having a primary winding, asecondary winding and a biasing winding, means for supplying to saidbiasing winding a constant direct current, means for supplying to saidprimary winding a current proportional to the current induced in saidfield winding after closure of said switching means, a second relayresponsive to a predetermined flow of current in said secondary windingfor causing opening of said switching means to disconnect said fieldwinding from said direct current source, and means for preventingoperation of said second relay in response to transient currentsproduced in said field winding upon connection of said field winding tosaid direct current source.

6. In a system comprising a synchronous motor having an armature windingand a field winding, a source of alternating current, a source of directcurrent, and means for connecting said armature winding to said sourceof alternating current, the combination of switching means forconnecting said field winding to said directcurrent source, a firstrelay responsive to the current induced by said armature winding in saidfield winding prior to closure of said switching means for causingclosure of said switching means to connect said field winding to saidsource of direct current, a current transformer having a primarywinding, a secondary winding and a biasing winding, means for supplyingto said biasing winding a constant direct current, means for supplyingto said primary winding a current proportional to the current induced insaid field winding after closure of said switching means, a second relayhaving a coil responsive to current of predetermined magnitude in saidsecondary winding for causing said switching means to disconnect saidfield winding from said direct current source, and contact means forshort circuiting said coil for a predetermined time after connection ofsaid field winding to said direct current source to prevent operation ofsaid second relay in response to transient currents induced in saidfield winding upon connection of said field winding to said directcurrent source.

'7. In a system comprising a synchronous motor having an armaturewinding and a field winding, a source of alternating current, a sourceof direct current, and means for connecting said armature winding tosaid source of alternating current, the combination of switching meansfor connecting said field winding to said direct current source, a firstrelay responsive to the current induced by said armature winding in saidfield winding prior to closure of said switching means for causingclosure of said switching means to connect said field winding to saiddirect current source, a current transformer having a primary winding, asecondary winding and a biasing winding, means for supplying to saidbiasing Winding a constant direct current, means for supplying to saidprimary winding a current proportional to the current induced in saidfield winding after closure of said switching means, a second relayresponsive to current of predetermined magnitude flowing in saidsecondary winding for causing said switching means to disconnect saidfield winding from said direct current source, and means including anelement of said switching means responsive to closure of said switchingmeans for rendering said second relay unresponsive to said secondarycurrent for a predetermined time following closure of said switchingmeans.

8. In a system comprising a synchronous motor having an armature windingand afield winding, a source of alternating current, a source of directciu'rent, and means for connecting said armature winding to said sourceof alternating current, the combination of switching means forconnecting said field winding to said direct current source, a firstrelay responsive to the current induced by said armature winding in saidfield winding for controlling closure of said switching means to connectsaid field winding to said direct current source, a current transformerhaving a primary winding, a secondary winding and a biasing winding,means for supplying a constant direct current to said biasing winding,means for supplying to said primary winding a current proportional tothe current induced in said field winding after closure of saidswitching means, a second relay responsive to current of predeterminedmagnitude flowing in said secondary winding for causing opening of saidswitching means to disconnect said field winding from said directcurrent source, and a third relay energized in response to closure ofsaid switching means for rendering said second relay unresponsive tocurrent fiow in said secondary winding for a predetermined timefollowing closure of said switching means.

9. In a system comprising a synchronous motor having an armature windingand a field winding, a source of alternating current, a source of directcurrent, and means for connecting said armature winding to said sourceof alternating current, the combination of switching means forconnecting said field winding to said direct current source to supplysaid field winding with direct current, a first relay responsive to thecurrent induced in said field winding for controlling closure of saidswitching means to connect said field winding in circuit with saiddirect current source, a reactor having a magnetic core and a currentwinding connected in said field winding circuit, a second relayconnected across said current winding and responsive to said inducedcurrent after closure of said switching means for causing opening ofsaid switching means to disconnect said field winding from said directcurrent source, and a rectifier connected in series with said secondrelay across said current winding for preventing energization of saidsecond relay by said direct current.

10. In a system comprising a synchronous motor having an armaturewinding and a field winding, a source of alternating current, a sourceof direct current, and means for connecting said armature winding tosaid source of alternating current, the combination of switching meansfor connecting said field winding to said direct current source tosupply direct current to said field winding, a first relay responsive tothe current induced by said armature winding in said field winding priorto closure of said switching means for causing closure of said switchingmeans to connect said field winding in circuit with said direct currentsource, a reactor having a magnetic core and a current winding connectedin said field winding circuit, a second relay connected across saidcurrent winding and responsive to said induced current after closure ofsaid switching means for causing opening of said switching means todisconnect said field winding from said direct current source, and acapacitor connected in series with said second relay across said currentwinding for preventing energization of said second relay by said directcurrent.

11. In a system comprising a synchronous m0- tor having an armaturewinding and a field winding, a source of alternating current, a sourceof direct current, and means for connecting said armature winding tosaid source of alternating current, the combination of switching meansfor connecting said field winding to said direct current source tosupply direct current to said field winding, a first relay responsive tothe current induced by said armature winding in said field winding priorto closure of said switching means for causing closure of said switchingmeans to connect said field winding in circuit with said direct currentsource, a reactor having a magnetic core and a current winding connectedin said field winding circuit, a second relay connected across saidcurrent winding and responsive to said induced current after closure ofsaid switching means for causing opening of said switching means todisconnect said field winding from said direct current source, arectifier connected in series with said second relay across said currentwinding for preventing energization of said second relay by said directcurrent, and means including an element of said reactor for preventingsaturation of said core by said direct current.

12. In a system comprising a synchronous motor having an armaturewinding and a field winding, a source of alternating current, a sourceof direct current, and means for connecting said armature winding tosaid source of alternating current, the combination of switching meansfor connecting said field winding to said direct current source tosupply direct current to said field winding, a relay responsive to thecurrent induced by said armature winding in said field winding prior toclosure of said switching means for causing closure of said switchingmeans to connect said field winding in circuit with said direct currentsource, means for causing opening of said switching means to disconnectsaid field winding from said direct current source including anelectroresponsive device having a magnetic core and a current windingconnected in circuit with said field winding circuit tobe responsive tocurrent induced by said armature winding in said field winding afterclosure of said switching means, and means including an element of saiddevice for preventing saturation of said core by said direct current.

TI-IADDEUS F. BELLINGER.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,150,662 Swanson Mar. 14, 19392,150,664 Swanson Mar. 14, 1939 2,407,121 Winter Sept. 3, 1946 2,478,693Herziger Aug. 9, 1949 2,504,812 Daugert Apr. 18, 1950 2,530,997Schaelchin Nov. 21, 1950

